In a pneumatic tire as a typical example of the rubber article, particularly a tire used under a relatively low internal pressure for use in passenger car, light truck or the like, organic fiber cords such as polyester cord, rayon cord, nylon cord and the like are used as a reinforcing member of a carcass forming a skeleton of the tire. Since a moderate ride comfort is particularly required in the passenger car tire, there are adopted organic fiber cords having a low tensile stress (modulus) at a constant elongation.
Since the organic fiber is relatively low in the modulus and such a modulus is lowered through a high temperature treatment in the vulcanization of the tire, there is a disadvantage that it is required to conduct cooling while feeding air into the tire to hold a constant internal pressure or so-called post cure inflation (PCI) just after the completion of the vulcanization step for preventing shrinkage of the cord.
And also, it is required to subject the organic fiber to an adhesion treatment for obtaining an adhesion to rubber, so that there is a problem that the productivity of the tire is poor.
Furthermore, there are problems in the adhesion durability between organic fiber and rubber and the heat resistance of the organic fiber itself under a higher temperature. For example, it is feared to cause peeling in the adhesion between cord and rubber or fusion of the cord due to heat generation accompanied with high-speed running.
In this connection, steel cords are high in the thermal stability and are advantageous in view of the change of modulus and the adhesion durability, so that it is attempted to apply the steel cord as a carcass ply cord in the passenger car tire.
Since the modulus of the steel cord is too high, however, there is a disadvantage that the degradation of the ride comfort can not be avoided in such a type of the tire that the deformation of the sidewall portion is large as in the passenger car tire.
And also, the steel cord is very high in the compression modulus as compared with the organic fiber cord. Therefore, when the steel cord is applied as a carcass ply cord in the passenger car tire used under a relatively low internal pressure, it can not bear to repetitive input of compression strain typified by buckling deformation generated in the cornering at, for example, a large steering angle and hence the breakage of the cord is caused. This is a large obstacle in the application of the steel cord to the carcass ply.
On the other hand, a motorcycle tire is essentially different in the cornering characteristic from a four-wheeled vehicle tire, so that the development of the motorcycle radial tire delayed. That is, the cornering of the motorcycle tire is realized by inclining the tire inward in the cornering to give a camber angle and producing a camber thrust in accordance with the magnitude of the camber angle. On the contrary, the cornering of the four-wheeled vehicle tire is based on a cornering force produced in accordance with the magnitude of a slip angle given to the tire through an operation of a handle without inclining the tire. Therefore, a profile shape of a tread considerably differs between the motorcycle tire and the four-wheeled vehicle tire. In the motorcycle tire, the tread is an arc shape extending between a pair of sidewalls up to positions corresponding to a maximum width of the tire at a widthwise section of the tire and can ensure a ground contact shape of not less than a certain value even at a side end region of the tread as the camber angle increases. In the motorcycle tire having such a tread form, even if the carcass is simply rendered into a radial carcass, the rigidity of the side portion is not ensured, so that the application of the radial structure is delayed as compared with the four-wheeled vehicle tire. Under such a circumstance, various techniques are developed for establishing the radial structure in the motorcycle tire.
For instance, in order to ensure the rigidity of the side portion in the motorcycle tire having a bias structure, a carcass comprised of at least two plies each containing cords arranged at an inclination angle of about 30-40° with respect to an equatorial plane of the tire is applied so as to cross the cords with each other between the plies. In this case, however, the bending rigidity of the tread portion becomes higher, so that a straight-running stability is obstructed.
In order to solve the above problem, JP-B-7-41764 discloses that an excellent straight-running stability is given to the motorcycle tire having a radial structure by constructing a belt with steel cords arranged substantially in parallel to the equatorial plane of the tire. Moreover, the term “straight-running stability” is defined as a convergence to external disorder during the running.
When the steel cords are arranged substantially in parallel to the equatorial plane of the tire, it newly causes a problem that the cords in the belt does not bear to the repetitive inputs of compression strain typified by the bending deformation generated in the tread portion during the running of the tire under loading and finally the breakage of the cord is generated. That is, the conventional steel cord is relatively high in the compression modulus, so that even if the compression strain applied to the cord is small, the buckling is generated in a part of the cord. If compression input is further applied to the cord, the bending deformation is progressed in such a buckled portion to finally cause fatigue breakage.
On the other hand, JP-A-4-232035 discloses that in the motorcycle radial tire having a belt containing steel cords arranged substantially in parallel to the equatorial plane of the tire, steel cord having a total elongation at break of 4-8% is used as a belt cord. And also, Japanese Patent No. 2935481 discloses that the elongation of the belt cord is defined under various loads in the motorcycle radial tire having a belt containing steel cords arranged substantially in parallel to the equatorial plane of the tire.
In these publications, the belt cord is selected from a viewpoint of tire production or as expected for improving the wear resistance of the tread portion. Since the selected cords indicate a high elongation, they are considered to advantageously act for controlling the buckling deformation when compression strain is generated in the steel cord as mentioned above.
However, these cords have a double strand construction obtained by further twisting plural strands each comprised of filaments twisted at a short pitch or subjected to forming, so that the cord production step becomes complicated and the productivity is poor as compared with a cord of a single strand construction completely produced at, for example, one twisting step and hence there is retained a problem in the cost.
Moreover, when the steel cord is applied to the belt, the rigidity is increased, so that the steering stability is improved but vibrations input to the tread portion are hardly absorbed and hence it tends to damage the vibration absorbability.